One known type of static read/write memory cell is a high-density static random access memory (SRAM). A static memory cell is characterized by operation in one of two mutually-exclusive and self-maintaining operating states. Each operating state defines one of the two possible binary bit values, zero or one. A static memory cell typically has an output which reflects the operating state of the memory cell. Such an output produces a "high" voltage to indicate a "set" operating state. The memory cell output produces a "low" voltage to indicate a "reset" operating state. A low or reset output voltage usually represents a binary value of zero, while a high or set output voltage represents a binary value of one.
A static memory cell is said to be bistable because it has two stable or self-maintaining operating states, corresponding to two different output voltages. Without external stimuli, a static memory cell will operate continuously in a single one of its two operating states. It has internal feedback to maintain a stable output voltage, corresponding to the operating state of the memory cell, as long as the memory cell receives power.
The two possible output voltages produced by a static memory cell correspond generally to upper (V.sub.CC internal-V.sub.T) and lower (V.sub.SS) circuit supply voltages. Intermediate output voltages, between the upper (V.sub.cc -V.sub.T) and lower (V.sub.SS) circuit supply voltages, generally do not occur except for during brief periods of memory cell power-up and during transitions from one operating state to the other operating state.
The operation of a static memory cell is in contrast to other types of memory cells such as dynamic cells which do not have stable operating states. A dynamic memory cell can be programmed to store a voltage which represents one of two binary values, but requires periodic reprogramming or "refreshing" to maintain this voltage for more than very short time periods.
A dynamic memory cell has no internal feedback to maintain a stable output voltage. Without refreshing, the output of a dynamic memory cell will drift toward intermediate or indeterminate voltages, resulting in loss of data. Dynamic memory cells are used in spite of this limitation because of the significantly greater packaging densities which can be attained. For instance, a dynamic memory cell can be fabricated with a single MOSFET transistor, rather than the six transistors typically required in a static memory cell. Because of the significantly different architectural arrangements and functional requirements of static and dynamic memory cells and circuits, static memory design has developed along generally different paths than has the design of dynamic memories.
Implementing a static memory cell on an integrated circuit involves connecting isolated circuit components or devices, such as inverters and access transistors, through specific electrical paths. When fabricating integrated circuits into a semiconductor substrate, devices within the substrate must be electrically isolated from other devices within the substrate. The devices are subsequently interconnected to create specific desired circuit configurations.
One common technique for isolating devices is referred to as LOCOS Isolation (for LOCal Oxidation of Silicon), which involves the formation of a semi-recessed oxide in the non-active (or field) areas of the bulk substrate. Such oxide is typically thermally grown by means of wet oxidation of the bulk silicon substrate at temperatures of around 1000.degree. C. for two to six hours. The oxide grows where there is no masking material over other silicon areas on the substrate. A typical masking material used to cover areas where field oxide is not desired is nitride, such as Si.sub.3 N.sub.4.
However, at the edges of a nitride mask, some of the oxidant also diffuses laterally immediately therebeneath. This causes oxide to grow under and lift the nitride edges. The shape of the oxide at the nitride edges is that of a slowly tapering oxide wedge that merges into a previously formed thin layer of pad oxide, and has been termed as a "bird's beak". The bird's beak is generally a lateral extension of the field oxide into the active areas of devices. Further process steps etch away part of the bird's beak oxide.
The threshold voltage (V.sub.T) of a MOS transistor determines the requirement for turning the MOS transistor on or off. Therefore, it is important to be able to adjust the threshold voltage in designing the MOS transistor. One common method of controlling threshold voltage is through the use of ion implantation (e.g., boron implantation). Because very precise quantities of impurity can be introduced using ion implantation, it is possible to maintain close control of V.sub.T. A shallow boron implant into the p-type substrate of an n-channel transistor will make the V.sub.T more positive with increasing dose.
The threshold voltage V.sub.T of a MOS transistor is also affected by gate dimensions. If channel length is long, the influence of the drain and source junctions on the quantity of charge in the channel is minimal. On the other hand, as channel length decreases and approaches the dimensions of the widths of the depletion regions of the source and drain junctions, the depletion regions become an increasing part of the channel-depletion region. Some of the channel-depletion region charge is linked to the charge in the depletion region in the source and drain structures instead of being linked to gate charge. The channel region is depleted in part without any influence of gate voltage. Therefore, because some of the channel is depleted without a gate bias, less gate charge is necessary to invert the channel in short channel devices than in a long channel device of comparable substrate doping.
To be able to establish slightly positive V.sub.T values, such as under one volt, in long-channel NMOS transistors with lightly doped substrates, it is necessary to increase the doping concentration at the surface of the channel which is done by a boron implant, typically through the sacrificial oxide or gate oxide.
There is a parameter in a static memory cell called beta ratio, which is approximately equal to: EQU [(pulldownW/pulldownL)/(accessW/accessL)]
where pulldownW is the effective electrical width of the active area of a pulldown transistor in the static memory cell; where pulldownL is the effective electrical length of the gate of the pulldown transistor in the memory cell; where accessW is the effective electrical width of the active area of an access transistor in the static memory cell; and where accessL is the effective electrical length of the gate of the access transistor in the static memory cell. This beta ratio is required to be above a predetermined value, such as 3.0, for stable operation of the static memory cell.
It is, of course, desirable to reduce the size of a static memory cell. Typically, to minimize the cell size, the accessW and pulldownL are set at the minimum values as defined by the process capability. Thus, it is necessary to increase accessL and/or pulldownW to maintain an acceptable beta ratio.